So yesterday I'm heading home from a beach run and the xj just dies. I pull over check for fuel and have it, check for spark and have none... Start checking the obvious and find that the coil wire is very badly corroded at the coil end and the wire is shot (crumbled when I pulled it off) I think cool, I found the problem. So I pulled a plug wire off one of the cylinders and figured I'd use it as my coil wire and limp on home on 5 cylinders... Put the wire on and it still wont fire. Wait about 20-30 minutes and it fires up! Close the hood and get started and it shuts back down in about a mile... Sit and wait for it to cool again and it starts back up and we repeat this 5-6 times... At that point i'm thinking its the VSS since it seemed to idle forever beside the road but as soon as I put it in gear and start rolling good it dies. ( I know the vss is wired into the ignition and its only active at above 180 degrees and about 1,500 rpm so it seems to fit the scenario) I end up pulling it on home last night but when I get it here It won't fire even though its cooled down below the vss thresholds so I start doubting that solution.
I get up this morning and she fires right up! Still running on 5 cylinders, so I swap in a new coil wire and get it running smooth again. I let it idle for about 30 minutes to get up to temp good and make sure its not just the coil overheating and going bad. Go back out and shes still running so I drive it around for a bit and after a few miles its doing the same thing again... The first few times it started back sooner with the loss of fire seeming to last longer the more it did it. At this point Im thinking maybe it is the coil going bad since I know they have a tendency to act up when hot and are on the way out but then I notice that when I loss fire I also loss my speedo and It (the coil) doesn't explain why it will idle forever but cut off when you're driving it.... Would love some input from you guys!! What do you think?? Possibly CPS, Coil, VSS?????

I just got back from driving it again and it cut off repeatedly though sporadic (Run a minute this time, 2-3 the next, etc) finally get it home and leave it idling in the yard and after about 30 minutes shes still running fine but I know if I get in and start driving shell die again...

 

Sounds like the crank position sensor due to the fact it would idle fine on the side of the road but would die under load. cheap part to fix too

 

Would the cps cause me to lose signal from the vss?? (the speedo dying when I lose fire)

Maybe I can find a ignition schematic and check it out on line.... Funds are really tight at the moment and I'm trying to be fairly certain before purchasing any parts, but I realize its still kind of a guessing game Thanks for the input!!

 

Sounds like a crank sensor, classic symtoms

 

Definitely sounds like the CPS. Before just replacing the part though, check the connection itself. This is often the culprit rather than the CPS itself. Unplug and plug it back in a couple times. If this helps, a good (and industry standard) fix is to simply solder the wires, bypassing the connection.

 

Wouldn't the cps still show symptoms at idle though???

 

What do you mean showing symptoms at idle?

 

If the connection for the CPS is bad you most likely won't get symptoms of a "bad CPS" until vibrations while driving down the road cause the connection to move around and make the CPS stop working. At idle the connection may be fine due to it not moving around.

 

Mine never show any symptoms at an idle only when it got rolling did it act up. New cps and now no problem.

 

Well it'll idle perfectly all day long and only dies while driving.... Seems to be somewhat heat related since it starts back after a quick cool down...

If the cps was shot I'd have no fire anytime. If was going bad sporadically I'd think it wouldn't only happen while driving, it should also die while at an idle which it doesn't.

I've checked the cps connections and they're all good. Even tapped it a few times while idling to imitate a bump while driving and it did fine. I'm not saying its for sure not the cps but I just don't get why it wouldn't die at idle if it was...

 

Maybe not enough heat generated to open the cicuit. Also I have seen a bad CPS and still get a random spark from the pcm because it see's the cam moving....

 

Thanks for all the help guys! I think I've got it fixed Just got back from driving it around for over an hour with no problems. I was pretty sure from the way it was acting it wasn't the cps and my mind kept wanting to think vss since the speedo would die with the fire... But it was too sporadic for a vss... Anyway I decided to check out the vss and the associated wiring and found the power wire from the speedo was rubbed through to the wire were it was touching the ds knuckle... (my fault for not securing it good enough during a tc swap a few months back) Anyway, I'm guessing it was hitting and grounding out the wire and thus shorting out the vss.. This would explain why it would only happen when moving and why the cooldown helped (allowing the vss to reset).